1. Field of the Invention
The present invention relates to liquid crystal display devices, and particularly to electrode arrangements for such devices.
2. Description of the Prior Art
Various types of liquid crystal displays are known which convey information in either only analog form or only digital form. The applicant's U.S. Pat. No. 3,820,875, which issued on June 28, 1974, discloses an analog type of scanner device having a field-effect light-scattering dielectric (e.g., a liquid crystal material) disposed between a pair of electrically insulative plates or substrates. The inner surface of one of the plates supports a resistor of predetermined length and configuration; the inner surface of the other plate supports a substantially nonresistive conductor, facing the resistor. At least one of the one plate and resistor and the other plate and conductor are transparent, so that the dielectric is visible through at least one of the plates.
A first voltage difference applied to the ends of the resistor produces a voltage gradient along the length of the resistor. Selective adjustment of a second voltage applied to the conductor to change the location on the one plate where the gradient voltage equals the conductor voltage renders the dielectric at that location visually distinguishable from the dielectric in the rest of the space between the plates. The patent discloses two different resistor patterns. The first is a continuous zig-zag pattern of spaced parallel lines, with successive adjacent pairs of the lines being joined together at alternate ends to form a rectangular serpentine array. A potential difference applied to the two ends of the array produces a voltage gradient that is a continuously varying function along the length of the zig-zag resistive pattern, from one end of the array to the other, so that any point located on the pattern is defined by a unique voltage.
In the second pattern, a straight resistor element extends along one edge of a plate, and a number of parallel spaced-apart low resistance conductor strips extend perpendicularly from the resistor across the plate, forming a rectangular array. A potential difference applied to the opposite ends of the resistor element in this arrangement also produces a continuous voltage gradient along the length of the element. Since the individual conductor strips contact the resistive element at different discrete voltage points along its length, each conductor strip will be defined by a unique voltage that differs from the voltage of a neighboring strip by the voltage difference between adjacent strip connection points on the resistive element.
The patent discloses a three-plate embodiment that includes a first plate having resistor patterns of the second type on both sides, but with the parallel conductor strips on one side extending in a direction perpendicular to the parallel conductor strips on the other side. The first plate is sandwiched between second and third plates, spaced from the respective faces of the first plate and each having a substantially nonresistive conductive film on the inner surface facing the respective series of parallel conductor strips. The spaces between the plates are filled with a field effect light scattering dielectric.
A potential difference impressed on the ends of either resistor and an intermediate voltage applied to the facing conductive film produce a linearly extending visually distinguishable region in the dielectric between them at a location where the potential of a conductor strip equals the voltage applied to the facing conductive film. If the dielectric is a liquid crystal, the visually distinguishable region normally is a transparent region. Since the transparent lines formed in the dielectrics on the opposite sides of the one plate are perpendicular to each other, only their intersection is transparent to light passing through both dielectrics. Thus a window is created at the intersection that appears as a visually distinguishable spot. When varying voltages are applied to the conductive films on the second and third plates, the spot can be selectively scanned over the display.
U.S. Pat. No. 4,493,531 of Bohmer et al., issued on Jan. 15, 1985, introduces several alternative embodiments of the field-sensitive optical display devices described above. These further embodiments include further layers, including polarizing films, so that the devices can produce various analog patterns such as a movable dot, intersecting lines, and a pivoting line.
There are also well known types of liquid crystal digital displays that employ patterns of isolated or discrete elements that are individually addressed by external circuitry. An example is the common digital watch display format that uses a rectangular arrangement of seven segmental elements to create numbers and letters by addressing selected ones of the seven elements. Another example is a variant of the three-plate arrangement described above that has a first series of closely spaced parallel conductive strips or lines on one side of an insulative substrate and a second series of closely spaced parallel conductive lines on the other side of the substrate. Instead of connecting the individual conductive lines of each series to a resistor having a terminal at each end, each of the lines has a separate connector at an edge of the substrate to permit the lines to be individually addressed to form a dot matrix display.
Typically the pattern elements of a liquid crystal display device, whether analog or digital, are created by depositing an appropriate material, for example, indium-tin oxide, on the plate. Once a pattern is deposited, its type is fixed; it cannot be changed from a discrete point voltage type to the discrete line voltage type, or vice-versa. In addition, to operate a digital display device in analog mode, for example, it would be necessary to connect external resistors between the isolated elements of the digital display. This would negate the simplicity and minimum number of external connection terminals that are a feature of the analog display devices.